Recently, there are seen many cases where water-soluble dyes containing sulfonate group are utilized in the field of information recording materials.
In most of these cases, as the sulfonate, sodium sulfonate is used. This is not only because the sodium salt is inexpensive but also because most of the dyes, though they are used in the field of electronic materials, have been diverted from the commercially available dyes for fibers.
When water-soluble dyes containing sodium sulfonate group (hereinafter abbreviated to "sodium type of dyes") are used, for example, as an ink composition for ink jet recording, heretofore, not a few dyes have proved unfit for practical use because of their insufficiency in solubility, especially the solubility in glycols that may be added as liquid medium, in spite of the fact that they are excellent in the aspect of qualities such as color tone, light fastness, etc. However, with regard to these dyes, it was found that when the sodium salt is replaced by lithium salt or ammonium salt, their solubilities in glycols and amides can be remarkably improved making it possible to obtain an ink composition having a dye concentration satisfactory from the practical viewpoint.
As the conventional process for preparation of the water-soluble dyes containing lithium sulfonate group or ammonium sulfonate group (hereinafter, referred to as "lithium type of dyes" and "ammonium type of dyes", respectively), there are known a process of separation by acid and a process of separation by salt (salting out process).
However, in the former process, the dye deposits in most of cases and usually as the deposited dye is considerably difficult to filter, it takes a long period of time for the separation, and moreover, as the wet cake of the separated dye contains a considerable amount of acid therein, a large amount of expensive lithium hydroxide or ammonium hydroxide is required for the neutralization. Also, by the operation being carried out under a strongly acidic condition the materials of the machinery and apparatus are inevitably greatly restricted.
In the latter process, usually by adding LiCl, NH.sub.4 Cl, etc. to an aqueous solution of a water-soluble dye containing sodium sulfonate group, i.e., by salting out, there are obtained the crystals of Li salt, NH.sub.4 salt, which are recovered by filtration, and after washing, are required to be redissolved and then purified by desalting. Thus, in this process, not only are the expensive LiCl, NH.sub.4 Cl, etc. needed in large quantities, but also the operations such as washing, redissolving, purification by desalting, etc. are troublesome and industrially disadvantageous.